The present invention relates to an MOCVD (Metal Organic Chemical Vapor Deposition) system used in semiconductor processes, and particularly to a system for forming a metal oxide film, such as a tantalum oxide (Ta.sub.2 O.sub.5) film, on a target substrate, while using as a liquid raw material a metal organic compound that is in a liquid phase at room temperature (about 20.degree. C.). The term "semiconductor process" used herein includes various kinds of processes which are performed to manufacture a semiconductor device or a structure having wiring layers, electrodes, and the like to be connected to a semiconductor device, on a target substrate, such as a semiconductor wafer or an LCD (Liquid Crystal Display) substrate, by forming semiconductor layers, insulating layers, and conductive layers in predetermined patterns on the target substrate.
In order to manufacture semiconductor devices, film formation and pattern etching are repeatedly applied to a semiconductor wafer. As semiconductor devices are increasingly highly miniaturized and integrated, demands on film formation become more strict. For example, very thin insulating films, such as capacitor insulating films and gate insulating films are still required to be thinner and to be more insulating.
Conventionally, silicon oxide films and silicon nitride films are used as the insulating films. In recent years, however, it has been proposed to form the insulating films from materials having more excellent insulating properties, such as metal oxides, e.g., tantalum oxide (Ta.sub.2 O.sub.5), or high-dielectric or ferroelectric bodies containing two metal elements or more, e.g., (Ba,Sr)TiO.sub.3, i.e., BST. These films can be formed by means of MOCVD, i.e., using vaporized metal organic compounds.
Jpn. Pat. Appln. KOKAI Publication No. 2-283029 discloses an MOCVD technique for forming a tantalum oxide film. In the technique disclosed in this publication, a metal (tantalum) alkoxide, such as Ta(OC.sub.2 H.sub.5).sub.5 (pentoethoxytantalum) is used as a liquid raw material. The liquid raw material is made to bubble by e.g., nitrogen gas, and is supplied to a process chamber preset to have a vacuum atmosphere. The supplied raw material is decomposed to offer a film forming material on the surface of a semiconductor wafer W heated to a process temperature of, e.g., 400.degree. C. With this film forming material, a tantalum oxide (Ta.sub.2 O.sub.5) film is formed on the surface of the semiconductor wafer W by means of deposition.
In this system, however, it is difficult to control the supply rate of the raw material with a high accuracy. This is because the liquid raw material is supplied by means of bubbling, and thus the flow rate of the liquid raw material is hardly accurately controlled. As a result, the supply rate of the raw material finally supplied to the process chamber shifts from the target value, thereby lowering reproducibility in the film thickness and quality of a formed CVD film.
Jpn. Pat. Appln. KOKAI Publication No. 10-79378 discloses another MOCVD technique for forming a tantalum oxide film, developed in light of the above described problem. In the technique disclosed in this publication, a liquid raw material the same as that of the former publication is delivered by the pressure of an inactive gas. The liquid raw material is supplied, while its flow rate is being controlled, and then is vaporized and turned into a process gas. The process gas is supplied to a process chamber and used for forming a CVD film on a semiconductor wafer.
FIG. 5 is a structural diagram schematically showing a conventional MOCVD system disclosed in the KOKAI Publication No. 10-79378.
In this system, a liquid raw material 4, such as pentoethoxytantalum, is stored in a raw material tank 2 and is delivered by the pressure of a pressurized gas, such as He gas. The stored liquid raw material 4 is heated by a heater 6 to a temperature of, e.g., from 20 to 50.degree. C., at which the liquid raw material 4 can easily flow. The delivered liquid raw material 4 flows downstream, while its flow rate is being controlled by a flow control unit 8, and then is vaporized and turned into a process gas by an inactive gas, such as He gas, in a vaporizing unit 10.
The process gas flows in a gas supply line 14, which is wrapped by, e.g., a tape heater 12 for preventing the process gas from being turned back to liquid, and enters a film-forming unit 16. The process gas is supplied from a showerhead 19 into a process chamber 18, and then is decomposed to offer a film forming material on the surface of a semiconductor wafer W heated to a process temperature. With this film forming material, a tantalum oxide film is formed on the surface of the semiconductor wafer W by means of deposition.
The system of the KOKAI Publication No. 10-79378 can control the supply rate of the raw material more accurately than the former system. Depending on a situation, sometimes the latter system still lowers reproducibility in the film thickness and quality of a formed CVD film, sometimes as the case may be. Furthermore, the system includes heated members, such as the vaporizing unit 10, pipes, and so forth, other than the process chamber 18, and thus thermally influences the ambient environment.